a) Field of the Invention
This invention relates to a real image mode variable magnification finder optical system for use in photographic cameras, video cameras, etc.
b) Description of the Prior Art
In a real image mode finder optical system, a Porro prism is frequently used to erect an image for observation, for instance, as set forth in U.S. Pat. No. 4,906,078. This is because the Porro prism can be compactly constructed over optical systems for erecting images such as image rotators and relay optical systems. Such a Porro prism, however, has encountered the problems that in most cases, an intermediate image is formed by an objective lens system, in the vicinity of the entrance surface of the Porro prism integrally constructed in general, and hence an optical system rendering relatively long the back focal distance of the objective lens system, namely, the distance from the rearmost surface of the objective lens system to the position of the intermediate image, will increases the finder length from the entrance surface of the objective lens system to the exit surface of an ocular system.
Thus, an arrangement has been employed from the past such that the intermediate image formed by the objective lens system is positioned within the Porro prism so that the objective lens system is closer to a Porro prism system, and thereby the finder length is reduced.
However, the intermediate image needs to provide a field frame for determining the range of a visual field at its position and, where the intermediate image is positioned within the Porro prism, it is common practice that the Porro prism is divided into a plurality of prisms and the intermediate image is positioned in the resultant space to place the field frame thereat. Unless the back focal distance of the objective lens system is kept to a length adequate for the dividing manner of the prism, the intermediate image will enter the prism, the field frame cannot be disposed, and the finder optical system will become oversized. As such, the optical system involves the difficulties that the optimum range of the back focal distance of the objective lens system is inevitably decreased, which restricts the conditions in the design of the objective lens system and forms one of the causes for deteriorating optical performance.